Scab disease of potatoes (Solanum tuberosum) is caused by several species of Streptomyces. The pathogen was first isolated in 1890 in the state of Connecticut and named Oospora scabies based on its melanin production and gray spores borne in spiral chains. Thaxter, Aqric. Expt. Sta. Rept., 81:95 (1890). The organism was later renamed as Streptomyces scabies. Waksman et al., J. Bacteriol., 46:337-341 (1948). Taxonomy of the organism has changed significantly over the years as different isolates of Streptomyces causing scab disease on potatoes have been given different names.
S. scabies was eventually reevaluated and the original description of the organism was changed to include smooth spores and use of all ISP sugars. Elesaway et al., ACTA Microbiol. Acad. Sci. Hung., 26:311-20 (1979). At present, the taxonomy of S. scabies is based on morphological and biochemical characteristics. The confusion over Streptomyces taxonomy may be attributed to the organism's ability to produce sectors readily in culture and extreme sensitivity to physical variations of cultural conditions. Erikson, J. Gen Microbiol., 13:136-148 (1955).
A number of different strains causing potato scab in the eastern United States and Canada were isolated, determined to be the same species, and grouped as Streptomyces scabies. Lambert et al., Intl. J. Systematic Bact. 39:387-392 (1989). Data indicated that S. scabies formed a homogeneous group characterized by smooth gray spores borne in spiral chains, melanin production, and utilization of all ISP sugars. It was thus proposed to revive the name S. scabies for the group of organisms which caused common scab of potato. At present, the species S. scabies is not listed in Bergey's Manual of Determinative Bacteriology (8th edition), Buchanan et al., Williams and Wilkins Co., Baltimore, Md. (1974).
Besides S. scabies, scab diseases on potato tubers are caused by other species of Streptomyces, as for example, S. acidiscabies, and additional less virulent isolates of S. griseus, S. olivaceous, S. aureofaciens, S. flaveolus. Corbaz, Phytopathol., 51:351-360 (1964); Gordon et al., J. Gen. Microbiol., 50:223-233 (1968); and Hutter, Systematic der Streptomyceten, Karger, Basel (1967); Archuleta et al., Amer. Potato J., 58:385-392 (1981); Millard et al., Ann. App. Biol., 8:560-644 (1926).
Crops other than potatoes, as for example, radish, beet, carrot, rutabaga, parsnip, turnip, and the like, are susceptible to scab diseases. Scab disease is a major production problem that affects grade quality, but has a relatively small effect on total yield of tubers or storing ability (Hooker, Compendium of Potato Disease, pages 33-34, American Phytopathological Society, St. Paul, Minn. (1990)).
Four different types of scab lesions have been observed on infected tuber surfaces. Lesions on potato tubers range from small raised scab tissue around lenticels to large, deep pits (Schroth et al., Ecology of Root Pathogens, pp. 129-138 (1979); Millard et al., Ann. App. Biol., 13:580-644 (1926); Jones, Ann. App. Biol., 18:313-333 (1931) 1941; and Schall, J. Agr. Res., 69:169-196 (1944)). Russet scab lesions are formed in highly alkaline soil by an undescribed species of Streptomyces (Harrison, Amer. Potato J., 39:368-387 (1962)). A recent paper, Faucher et al., Plant Dis., 77:1217-1220 (1993), indicates russet scab in eastern Canada is caused by Streptomyces aureofaciens. Lesions from "acid scab" isolates are caused by Streptomyces acidiscabies. Manzer et al., LSA Experimental Station Technical Bulletin, 85 (1976); Lambert and Loria, Intl. J. Systematical Bacteriology, 39:387-392 (1989).
Streptomyces spp. are aerobic, gram-positive, soil-inhabiting, filamentous bacteria. The Streptomyces are different from most prokaryotes because they produce coherent, nonfragmenting colonies and undergo differentiation during colony development producing hyphae and spores. Chater and Merrick, Studies in Microbiology. Vol 1: Developmental Biology of Prokaryotes, pages 93-114, J. H. Parish, ed., University of California Press, Berkeley, Calif. (1979). It has been suggested the Streptomyces occupy an evolutionary position intermediate between eubacteria and lower fungi because of their filamentous growth habit, production of aerial spores, presence of reiterated DNA, and DNA repair mechanisms. Stonisfer et al., Proc. Natl. Acad. Sci., 82:1180-1183 (1985).
Heterokaryosis and a compatibility system controlling heterokaryon formation were demonstrated in S. griseus and S. cyaneus. Bradley et al., J. Bacteriol., 72:235-241 (1956); Bradley et al., Proceedings for the Society for Experimental Biology and Medicine, 99:476-478 (1958). Hyphal anastomosis of S. scabies has been studied from a cytological perspective. Gregory, Can. J. Microbiol., 2:649-655 (1956). Vegetative compatibility with respect to heterokaryon formation by pairing N-methyl-N'-nitro-N-nitrosoguanidine (NTG) generated auxotrophic mutant strains of Streptomyces spp. Lorang, "Heterokaryosis and Inhibitory Reactions Among Isolates of Streptomyces scabies Causing Scab on Potato", unpublished Masters thesis, University of Minnesota (1988). Heterokaryons were formed between isolates causing various common scab lesions. In some plant pathogenic fungi, determination of vegetative compatibility groups is useful since they are linked to symptom development and host specificities. Parmeter et al., Phytopatholovy, 59:1270-1278, (1969).
Another study described the inhibitory reactions among paired isolates of Streptomyces spp. from potato scab lesions. McQueen et al., J. Gen Microbiol., 126:427-442 (1981). In that study, plasmid DNA was found in four isolates. Further studies by Lorang (1988) showed that some inhibitory reactions were due to physical contact resembling the lethal zygosis-like reaction and others to antibiotic production. Lorang, "Heterokaryosis and Inhibitory Reactions Among Isolates of Streptomyces scabies Causing Scab on Potato," Masters thesis, University of Minnesota (1988). Streptomyces plasmids, such as SCP1, SCP2, and SCP2*, increased chromosomal recombination and have been physically characterized. Bibb et al., Mol. Gen. Genet., 154:155-156 (1977). It was shown that the isolates with these plasmids inhibited growth of plasmid-minus isolates of Streptomyces spp. and caused the lethal zygosis-reaction. Bibb et al., J. Gen. Microbiol., 126:427-442 (1981).
Some important aspects of pathogenesis by S. scabies have been elucidated by Lawrence et al., Phytopathology, 80(7):606-608 (1990). Two compounds, thaxtomins A and B, were found to be responsible for formation of symptoms typical of the common scab disease. These compounds were identified as 4-nitroindol-3-yl containing 2,5-dioxopiperazines and were obtained from cell-free extracts of scab lesions of field-grown and cultural tubers. These results were confirmed by Babcock, Eckwall and Schottel who also found that thaxtomins A and B were formed in a synthetic culture medium. Babcock et al., J. Gen. Microbiol., 139:1579-1586 (1993). It has been demonstrated a positive correlation between the pathogenicity of 28 S. scabies isolates and their ability to produce thaxtomin A. King et al., Amer. Potato J., 68:675-680 (1991)
Also associated with potato scab is the natural decline of the disease in soils repeatedly grown to potatoes for many years. Schneider, Suppressive Soils and Plant Disease, American Phytopathology Society, St. Paul, 88 pp. (1982). It was demonstrated that a biological factor was responsible for scab decline in these disease conducive soils. Application of 1% suppressive soil plus 1% alfalfa meal into disease conducive soils controlled the scab disease. Menzies, Phytopathology, 49:648-653 (1959).
In an attempt to identify the cause of scab suppression, Lorang isolated several Streptomyces isolates which produced antibiotics against isolates and reduced the number of scab lesions on radish roots in a greenhouse test and on potato tubers grown in a field test. Lorang, "Heterokaryosis and Inhibitory Reactions Among Isolates of Streptomyces scabies Causing Scab on Potato", Masters thesis, University of Minnesota (1988). It has also been reported that an antibiotic biofertilizer (swine feces) contained Streptomyces albidoflavus strain CH-33 that controlled the potato common scab disease. Hayashida et al., Agric. Biol. Chem., 52(10):2397-2402 (1988); Hayashida et al., Agric. Biol. Chem., 53(2):349-354 (1989). Various species of bacteria have been used in a process called "potato seed tuber bacterization" in which a 30% disease reduction was claimed. Tanii et al., Biological Control of Soil-Borne Plant Pathogens, Hornby, ed., C.A.B. International (1990). It was also reported that potato scab disease could be controlled by increasing soil acidity and application of fungicides to the soil. Davis et al., Americ. Potato J., 51:35-43 (1974); Nuget, Plant Disease Reporter, 40:428 (1956).
Crop rotation is a major means of minimizing scab. Scab has also been reported to be suppressed by incorporation of green manures that enhance microbial antagonists of the scab organisms, by excess irrigation practices during tuber formation, by chemical control using gypsum, sulfur, and PCNB (pentachloronitro benzene), or by soil fumigation. Rouatt et al., Can. J. Research, 28:140-152 (1950); Weinhold et al., Plant and Soil, 28(1):12-24 (1968); Lapwood, et al., American Phytopatholocy Society, at pages 123-129 (1975). It has been reported that scab incidence was correlated with soil pH, but was not correlated with calcium concentration in soil, healthy tuber periderm, or medulla tissue. Lambert and Manzer, Phytopathology, 81:632-636 (1991). In that study, scab lesion diameter was inversely correlated with tuber magnesium and manganese concentration.
The use of scab resistant potato cultivars is another method of disease control. In a study on population dynamics, it was indicated that the total population of Actinomycetes and S. scabies were greater on the tuber surface of the susceptible cultivar Chippewa than the resistant cultivar Superior, but soil and rhizosphere populations of these bacteria did not differ between cultivars. Keinath et al., Amer. Potato J., 68:515-524 (1989). In general, scab severity increased linearly with an increase in log.sub.10 of S. scabies inoculum (Keinath and Loria, Phytopathology, 79:681-687 (1991)).
The need to reduce our dependence on chemicals to control plant disease is recognized widely. The use of chemicals is expensive, and one traditional chemical approach using sulfur to maintain low soil pH limits the crops that can be grown in rotation with potatoes.
Attempts have been made to develop biological controls for plant diseases, but only a few methods have been successful. For example, A. radiobacter strain 84 which produces agrocin 84 was developed as a biological control against crown gall (Agrobacterium tumefaciens). Kerr, Plant Disease, 64:24-30 (1980). Another example of a biocontrol mechanism is the control of the American chestnut blight caused by Cryphonectria parasitica by use of vegetatively compatible strains of the pathogen into which double stranded RNA has been transferred to make them hypo. Kuhlman et al., Phytopathology, 74:659-664 (1984).
Therefore, an object of the invention is to develop a biological control for plant diseases such as scab disease in potatoes and other susceptible crops. Another object is to develop a method for inhibiting scab caused by Streptomyces spp. Yet another object is to develop a biological control for diseases caused by Fusarium, Rhizoctonia, Verticillium, Pythium, Clavibacter, and other like soil-borne plant pathogens.